Variation in load of accessories, such as a pump for power steering and a compressor for air conditioner, causes fluctuation in torque of internal combustion engine. In a case that the fluctuation in torque is tried to be restricted by correcting the intake air flow rate, there in a delay in response until the variation in the throttle position (or valve opening degree of idle speed control valve) emerges as a variation in the intake air flow rate, so that the fluctuation in torque (fluctuation in speed of engine) is hardly restricted in a high response by correcting the intake air flow rate.
U.S. Pat. No. 5,765,527 shows a method in which the fluctuation in torque is well restricted by correcting an ignition timing from a view point that a correction of the ignition timing in advance/retard direction emerges as a variation in torque in a high response.
Generally, the ignition timing of the internal combustion engine is adjusted in such a manner as to be close to MBT (Minimum spark advance for Best Torque) in which the torque and fuel economy are in the best conditions. In the case the ignition timing is too closed to the MBT, although the fluctuation in torque is tried to be restricted by compensating a decrement in torque due to the load of the accessories with the increment in torque of the ignition timing, the advance correction of the ignition timing hardly restrict the fluctuation in torque enough.
In the torque control method shown in U.S. Pat. No. 5,765,527, the intake-air-flow-rate control-value is offset in a torque increasing direction by the reserved torque, and this increment in torque is canceled by offsetting the ignition timing in a retard direction so that the required torque is obtained and the increment in torque is reserved to compensate the decrement in torque due to the load of accessory by advance correction of the ignition timing. When the ignition timing is offset in the retard direction, an estimated torque at the MBT is computed based on the intake air flow rate and an engine speed, and then the offset amount of the ignition timing in the retard direction is computed based on a ratio between the required torque and the estimated torque at the MBT.
However, if leak air passing through the throttle valve during idling is increased due to individual differences of the system, an increment of the estimated torque due to the leak air cannot be neglected. Thus, the ratio between the required torque and the estimated torque becomes small to increase the offset amount of the ignition timing in the retard direction, whereby the ignition timing may be retarded too much to deteriorate a fuel economy and to increase a temperature of exhaust gas.